SRAM Multiplexing Apparatus

ABSTRACT

An SRAM multiplexing apparatus comprise a plurality of local multiplexers and a global multiplexer. Each local multiplexer is coupled to a memory bank. The global multiplexer has a plurality of inputs, each of which is coupled to a corresponding output of the plurality of local multiplexers. In response to a decoded address in a read operation, an input of a local multiplexer is forwarded to a corresponding input of the global multiplexer. Similarly, the decoded address allows the global multiplexer to forward the input signal to a data out port via a buffer.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a divisional of U.S. patent application Ser. No.13/157,163, entitled “SRAM Multiplexing Apparatus,” filed on Jun. 9,2011, which application is incorporated herein by reference.

BACKGROUND

Modern electronic devices such as a notebook computer comprise a varietyof memories to store information. Memory circuits include two majorcategories. One is volatile memories; the other is non-volatilememories. Volatile memories include random access memory (RAM), whichcan be further divided into two sub-categories, static random accessmemory (SRAM) and dynamic random access memory (DRAM). Both SRAM andDRAM are volatile because they will lose the information they store whenthey are not powered. On the other hand, non-volatile memories can keepdata stored on them permanently unless non-volatile memories are exposedto an electrical charge. Non-volatile memories include a variety ofsub-categories, such as electrically erasable programmable read-onlymemory (EEPROM) and flash memory.

SRAM cells may comprise different numbers of transistors. According tothe total number of transistors in an SRAM cell, the SRAM cell may bereferred to as a six-transistor (6-T) SRAM, an eight-transistor (8-T)SRAM, and the like. SRAM cells are arranged in rows and columns. An SRAMcell is selected during either a read operation or a write operation byselecting its row and column. The row and column to be selected aredetermined by a binary code. For example, a 64Kb memory chip maycomprise a 16-bit binary code controlling the write and read operation.More particularly, the 16-bit binary code is split into two separate8-bit binary codes for selecting a row and a column respectively. The64Kb memory chip may further comprise a row decoder and a columndecoder. In response to an 8-bit code, the row decoder is able togenerate 2⁸ outputs, which comes to 256 outputs. Likewise, the columndecoder is able to generate another 2⁸ outputs. By enabling an outputfrom the row decoder and an output from the column decoder, an SRAM cellcan be selected from a memory cell matrix having 256 rows and 256columns.

In a read operation, the access time of detecting a logic state storedin a memory cell is a key performance index for a memory circuit. Themajor delay may result from bit line sensing due to the largecapacitance resulting from a large number of memory cells coupled to abit line. In order to reduce delay associated with bit line sensing,modern memory circuits may partition bit lines into two groups, namelylocal bit lines and global bit lines. As a result, a local bit line mayperform a fast read operation because the capacitance of the local bitline is reduced in comparison to that of the bit line in a memorycircuit without bit line partitioning.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a memory circuit comprising a two level multiplexingapparatus in accordance with an embodiment;

FIG. 2 illustrates in detail the schematic diagrams of the localinput/output (I/O) multiplexer, the global I/O multiplexer and thebuffer shown in FIG. 1; and

FIG. 3 illustrates a schematic diagram of a two level multiplexingapparatus and a memory circuit formed by a plurality of memory banks.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the variousembodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

The present invention will be described with respect to preferredembodiments in a specific context, an SRAM memory circuit comprising atwo level multiplexing apparatus. The invention may also be applied,however, to a variety of memory circuits.

Referring initially to FIG. 1, a memory circuit having two levelmultiplex apparatus is illustrated in accordance with an embodiment. Thememory circuit may comprise M memory banks, namely BANK1, BANK2, . . . ,BANKM. Each memory bank (e.g., BANK1) may comprise N pairs of local bitlines. An upper bit line (e.g., BL_U0) is coupled to an upper memorycluster, which may comprise a plurality of memory cells connected inparallel. On the other hand, a down bit line (e.g., BL_D0) is coupled toa down memory cluster, which may comprise a plurality of memory cellsconnected in parallel. As shown in FIG. 1, an upper bit line (e.g.,BL_U0) and a down bit line (e.g., BL_D0) are coupled to a senseamplifier (e.g., SA0). For a memory bank having N pairs of local bitlines, N sense amplifiers are employed to receive the data from thememory bank. More particularly, each sense amplifier receives two inputsignals from a local up bit line and a local down bit line respectively.Furthermore, the sense amplifier performs a NAND operation and generatesan output coupled to a corresponding input of the local input/output(I/O) multiplexer 102.

In accordance with an embodiment, a memory bank such as BANK1 maycomprise a plurality of memory cells (not shown) arranged in rows andcolumns. As known in the art, each memory cell may comprise two accessswitches whose gates are coupled to a word line. Furthermore, in a reador write operation, the memory cells arranged in one row are controlledby a same word line. More particularly, according to the decoded addressof a read or write control signal, a word line is set to high when therow of memory cells coupled to the word line will be accessed. The logichigh state at the word line turns on each memory cell's access switchesof the memory row to which the word line is coupled. As a result, a reador write operation can be performed through the turned on accessswitches.

The memory cells in a column of the memory bank BANK1 may be verticallyconnected to a local bit line (e.g., BL_D0). As shown in FIG. 1, in thememory bank BANK1, there may be N local down bit lines and N local upperbit lines. It should be noted that while FIG. 1 shows a single bit lineis connected to a column of memory cells, in order to achieve operation,both a bit-line (BL) and the inverse of BL ( BL) are employed to fulfillmemory access operations. A local down bit line and a correspondinglocal upper bit line are coupled to a sense amplifier, which generatesan output based upon the data on both the local down bit line and thelocal upper bit line. The output of the sense amplifier is furthercoupled to a local I/O multiplexer 102.

The local I/O multiplexer 102 has N inputs, each of which is coupled toa corresponding output of a sense amplifier. In response to a decodedaddress of a read operation, the local I/O multiplexer 102 may activatean internal logic circuit (not shown but illustrated in FIG. 2) coupledto an input. Because the rest of inputs of the local I/O multiplexer 102are not activated, the outputs coupled to inactivated inputs keep theirprevious logic states. Furthermore, the N outputs from the local I/Omultiplexer 102 are forwarded to a global I/O multiplexer 104. Thedetailed operation of the local I/O multiplexer 102 will be describedbelow with respect to FIG. 2. The global I/O multiplexer 104 is coupledbetween the local I/O multiplexer 102 and a buffer 106. In response to adecoded address, the global I/O multiplexer 104 selects an input from Ninputs sent from the M local I/O multiplexers 102. Furthermore, theglobal I/O multiplexer 104 forwards the selected input to the buffer106. The buffer 106 may employ a bus keeper (not shown but illustratedin FIG. 2) so that reliable data out can be read at the output of thebuffer 106. The detailed operation of the global I/O multiplexer and thebuffer 106 will be described below with respect to FIG. 2.

FIG. 2 illustrates in detail illustrative schematic diagrams of thelocal I/O multiplexer 102, the global I/O multiplexer 104 and the buffer106 shown in FIG. 1. The local I/O multiplexer 102 may comprise aplurality of logic cells (e.g., logic cell 202), each of which iscoupled to a local upper bit line and a local down bit line.Furthermore, each logic cell of the local I/O multiplexer 102 mayreceive a read selection signal decoded by a controller (not shown) froma read address. According to an embodiment, read selection signals fromYB0 to YBN are decoded from a read address. During a read operation,only one of the selection signals will be set to low. As a result, onlyone logic cell is enabled during a read operation and the correspondingdata read from a memory cell is forwarded to the corresponding globalbit line connected between the local I/O multiplexer 102 and the globalI/O multiplexer 104. In order to describe the detailed operation of thelocal I/O multiplexer 102, a logic cell 202 is used to describe how thelocal I/O multiplexer 102 performs a multiplexing operation.

The logic cell 202 comprises a negated AND (NAND) gate ND0, a firstp-type metal oxide semiconductor (PMOS) transistor ME0, a first n-typemetal oxide semiconductor (NMOS) transistor MD0 and a second NMOStransistor MF0. The logic cell 202 receives signals from the memory bankBANK1 via a local upper bit line BL_U0 and a local down bit line BL_D0.In addition, the logic cell 202 receives a control signal YB0 derivedfrom a decoded address. As shown in FIG. 2, the NAND gate ND0 has twoinputs coupled to the local upper bit line BL_U0 and the local down bitline BL_D0 respectively. The output of the NAND gate ND0 is coupled to aglobal bit line GBL0 via the first NMOS transistor MD0. The first NMOStransistor MD0 functions as a buffer to isolate the global bit line GBL0from the local bit lines.

Both the first PMOS transistor ME0 and the second NMOS transistor MF0are used to either disable or enable the sense amplifier ND0 so that thelogic cell 202 can perform a multiplexing function in response to thecontrol signal YB0. More particularly, when the control signal YB0 iskept high, the first PMOS transistor ME0 is turned off and the secondNMOS transistor MF0 is turned on. As a result, the NAND gate ND0 isdisconnected from the power supply VDD and the output of the NAND gateND0 is set to a logic low state. Consequently, the drain of the firstNMOS transistor MD0 stays at the previous logic state. On the otherhand, when the control signal YB0 is kept low, the first PMOS transistorME0 is turned on and the second NMOS transistor MF0 is turned off. As aresult, both the first PMOS transistor ME0 and the second NMOStransistor MF0 have no impact on the operation of the sense amplifierND0. The data read from the memory bank BANK1 is forwarded to the globalbit line GBL0 via the sense amplifier ND0 and the buffer transistor MD0.In sum, when the control signal YB0 of the logic cell 202 is set tohigh, the local upper bit line and the local down bit line coupled tothe logic cell 202 are not selected. In contrast, when the controlsignal YB0 of the logic cell 202 is set to low, the local upper bit lineand the local down bit line coupled to the logic cell 202 are selected.After a NAND operation through the sense amplifier ND0, the data on thelocal bit lines is forwarded to the subsequent stage as a global bitline signal.

The schematic diagram of the logic cell 206 (i.e., the Nth logic cell)is the same as that of the logic cell 202, and hence is not discussedherein in order to avoid repetition. As shown in FIG. 2, a plurality oflogic cells (e.g., logic cell 202) form the local I/O multiplexer 102. Acontroller (not shown) decodes a read address and generatescorresponding control signals for each logic cell. For a read operation,in response to a read address, only one control signal is set to a logiclow state. As a result, only a corresponding logic cell is enabled.Through the enabled logic cell, the data on the local bit line isforwarded to the global bit line connected to the output of the logiccell. An advantageous feature of having a local I/O multiplexer is thatonly one global bit line may have a logic state transition during a readoperation. A single global bit line logic state transition during a readoperation may reduce the total power consumption of a memory circuit incomparison with a memory circuit having multiple global bit line logicstate transitions during a read operation.

The global I/O multiplexer 104 comprises a plurality of logic circuits,each of which corresponds to a global bit line (e.g., GBL0). A logiccircuit 204 is used to describe the operation of the global I/Omultiplexer 104. The logic circuit 204 has an input coupled to theglobal bit line GBL0, an output coupled to the buffer 106 and a controlsignal input coupled to a control signal Dec[0], which is inverted fromthe control signal YB0. The logic circuit 204 comprises a first PMOStransistor MA0, a second PMOS transistor MP0, a first NMOS transistorMB0 and a second NMOS transistor MC0. The control signal Dec[0] is usedto either disable or enable the logic circuit 204 depending on the logicstate of the control signal Dec[0].

During a read operation, when Dec[0] is set to high, the second PMOStransistor MP0 is turned off and the first NMOS transistor MB0 is turnedon. As a result, the first PMOS transistor MA0 and the second NMOStransistor MC0 form an inverter. Such an inverter allows the global bitline GBL0 to be selected. As a result, the data on the global bit lineGBL0 is forwarded to the buffer 106. The global I/O multiplexer 104comprises a plurality of identical logic circuits. As described abovewith respect to the local I/O multiplexer 102, during a read operation,only one control signal is set to low in the local I/O multiplexer 102.Thus, only one control signal such as Dec[0] in the global I/Omultiplexer 104 is set to high because Dec[0] is the inverse signal ofYB0. As such, during a read operation, only one logic circuit of theglobal I/O multiplexer 104 is enabled.

The logic circuits of the global I/O multiplexer have their outputsconnected together. As described above, during a read operation, inresponse to a decoded address, only one logic circuit in the local I/Omultiplexer 102 and the corresponding logic circuit in the global I/Omultiplexer 104 are selected. The data at local bit lines coupled to theselected local I/O multiplexer input is forwarded to the buffer 106. Thebuffer 106 comprises an inverter B1 and a bus keeper. Both the inverterand the bus keeper are known in the art, and hence are not discussedherein.

FIG. 3 illustrates a schematic diagram of a two level multiplexingapparatus and a memory circuit formed by a plurality of memory banks. Inaccording with an embodiment, a memory circuit may comprise M memorybanks. As illustrated in FIG. 3, each memory bank may comprise a localI/O multiplexer having N logic circuits 202. Each global bit line iscoupled to the corresponding output of the local I/O multiplexer of eachmemory bank. For example, the global bit line GBL0 is coupled to thelogic circuit 202 in the memory bank BANK1 as well as the logic circuit302 in the memory bank BANKM. Each logic circuit (e.g., logic circuit202) may comprise an output NMOS transistor (e.g., the first NMOStransistor MD0). As a result, the global bit line GBL0 is coupled to MNMOS transistors. Similarly, the global bit line GBLN is coupled to MNMOS transistors. In consideration of the parasitic capacitance at theoutput of each NMOS transistor, the total capacitance loading of eachglobal bit line is equal to M times the parasitic capacitance of theNMOS transistor (e.g., MD0). In comparison with some memory circuithaving a single global bit line, by employing N global bit lines thecapacitance loading of each global bit line is reduced by a factor of N.

Although embodiments of the present invention and its advantages havebeen described in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the invention as defined by the appended claims.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

What is claimed is:
 1. A method comprising: performing a first logicoperation by a first sense amplifier, wherein the firs sense amplifierhas a first input coupled to a first bit line of a memory bank and asecond input coupled to a second bit line of the memory bank; inresponse to a decoded address of a read operation, selecting an outputof a first multiplexer, wherein the output of the first multiplexer iscoupled to a second multiplexer and an input of the first multiplexer iscoupled to the first sense amplifier; and in response to the decodedaddress, selecting, by the second multiplexer, an input from a pluralityof inputs coupled to the second multiplexer.
 2. The method of claim 1,further comprising: performing a NAND operation by the first senseamplifier, wherein the first sense amplifier is coupled between thememory bank and the first multiplexer.
 3. The method of claim 1,wherein: the first sense amplifier is a NAND gate.
 4. The method ofclaim 1, further comprising: forwarding, by the second multiplexer, aselected input to a buffer coupled to the second multiplexer.
 5. Themethod of claim 4, wherein the second multiplexer comprises: theplurality of inputs, each of which is coupled to a corresponding outputof the first multiplexer; and an output coupled to the buffer.
 6. Themethod of claim 1, wherein the first multiplexer comprises: a pluralityof inputs coupled to a plurality of bit lines; and a plurality of firstoutputs coupled to the second multiplexer.
 7. The method of claim 1,wherein the memory bank comprises: a first group of memory cellsarranged in columns, wherein each column is coupled to the first bitline; and a second group of memory cells arranged in columns, whereineach column is coupled to the second bit line.
 8. A method comprising:receiving, by a plurality of first level multiplexers, a first controlsignal derived from a decoded address, wherein the plurality of firstlevel multiplexers, each of which is coupled to a memory bank; inresponse to the first control signal, each first level multiplexerselecting an input from a plurality of inputs sent from a correspondingmemory bank; receiving, by a second level multiplexer, a second controlsignal derived from the decoded address, wherein the second levelmultiplexer is coupled to the plurality of first level multiplexers; andsending an output signal from the second level multiplexer to a buffer.9. The method of claim 8, further comprising: generating, by the buffer,an output at a data out port.
 10. The method of claim 8, wherein: thefirst level multiplexer comprises a plurality of first level logiccircuits, each of which is coupled to a first bit line, wherein during aread operation one first level logic circuit is enabled in response tothe first control signal; and the second level multiplexer comprises aplurality of second level logic circuits, each of which is coupled to acorresponding output of the plurality of first level multiplexers,wherein during a read operation one second level logic circuit isenabled in response to the second control signal.
 11. The method ofclaim 8, further comprising: generating the second control signalthrough inverting the first control signal.
 12. The method of claim 8,further comprising: during a read operation, activating an input of afirst level multiplexer based upon the first control signal.
 13. Themethod of claim 8, wherein the memory bank comprises: a first group ofmemory cells arranged in columns, wherein each column is coupled to afirst bit line; a second group of memory cells arranged in columns,wherein each column is coupled to a second bit line; and a senseamplifier having inputs coupled to the first bit line and the second bitline.
 14. The method of claim 13, further comprising: performing a NANDoperation through the sense amplifier, wherein the sense amplifier iscoupled between the memory cells and the first level multiplexer.
 15. Amethod comprising: receiving a decoded address during a read operation;performing a first multiplexing operation by selecting one memory celloutput from a plurality of memory cells based on the decoded address;and performing a second multiplexing operation by selecting an inputcorresponding to a selected input at the first multiplexing operationbased upon the decoded address.
 16. The method of claim 15, furthercomprising: deriving a first control signal from the decoded address;activating an input of a first multiplexer based upon the first controlsignal; generating a second control signal by inverting the firstcontrol signal; and activating an input of a second multiplexer basedupon the second control signal.
 17. The method of claim 15, furthercomprising: receiving a first data signal from a first bit line coupledto a first memory cluster of a memory bank; and receiving a second datasignal from a second bit line coupled to a second memory cluster of thememory bank.
 18. The method of claim 17, further comprising performing aNAND operation on the first data signal and the second data signal. 19.The method of claim 15, further comprising: generating an output datasignal at an output of a second level multiplexer; and forwarding theoutput data signal to a data out port via a buffer.
 20. The method ofclaim 15, further comprising activating a memory bank of a plurality ofmemory banks during the read operation.